A balanced-unbalanced conversion element that has one ½-wavelength resonator and two ¼-wavelength resonators formed on a dielectric substrate and performs balanced-unbalanced conversion has been suggested (see, for example, Patent Document 1).
FIG. 1 shows a configuration of a balun serving as a balanced-unbalanced conversion element according to the related art. A balun 101 is formed of a plurality of laminated dielectric substrates. This balun 101 has a ground electrode (not shown) on each of an upper lateral surface A and a lower lateral surface B, an unbalanced terminal (not shown) on a left lateral surface C, and two balanced terminals (not shown) on a right lateral surface D. On an illustrated upper surface of a substrate 105, an unbalanced pattern 102 is formed. The unbalanced pattern 102 is an electrode that constitutes a ½-wavelength resonator. A balanced pattern 103A and a balanced pattern 103B are formed on a dielectric substrate that is laminated on a back surface of this dielectric substrate 105. The balanced pattern 103A and the balanced pattern 1033 are electrodes that constitute different ¼-wavelength resonators.
The unbalanced pattern 102 is a substantially U-shaped electrode including parallel line portions 102A and 102B, a line portion 102C for connecting the line portions 102A and 102B, an lead electrode 102D to be connected to the ground electrode, and an lead electrode 102E to be coupled to the unbalanced terminal. Each of the balanced patterns 103A and 103B is a substantially I-shaped electrode pattern. The line portions 102A and 102B of the unbalanced pattern 102 face the balanced pattern 103A and the balanced pattern 103B through a first dielectric substrate, respectively.
In response to input of an unbalanced signal to the unbalanced terminal, this balun 101 converts the unbalanced signal into balanced signals and outputs a first balanced signal from one of the balanced terminals and a second balanced signal having a substantially opposite phase of the first balanced signal from the other balanced terminal.
Conversely, in response to input of balanced signals to the two balanced terminals, the balun converts the balanced signals into an unbalanced signal and outputs the unbalanced signal from the unbalanced terminal.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 10-290107
In general, a balance characteristic of a balanced-unbalanced conversion element is evaluated by a width of a frequency band in which a phase difference and an amplitude difference of two balanced signals converge to a predetermined range.
However, since a shape of the unbalanced pattern 102 and arrangement of the balanced patterns 103A and 103B are asymmetric in the balun 101 according to the related art, a frequency band that gives an appropriate balance characteristic is undesirably narrow.